The possibility that the green house effect due mainly to increases in the amount of CO2 in the atmosphere may be changing the climate of the earth has been pointed out in recent years. It has been also pointed out that the air pollutants, including CO2, NOx, and hydrocarbons, discharged from automobiles being used as moving means affect health. In terms of the protection against increases in the prices of energy resources such as oil, the preservation of the environment, or the protection against disasters, high expectations have been recently placed on hybrid vehicles, which are each a combination of an electric motor driven by electricity stored in a storage device and an engine and which are energy-efficient, electric vehicles, smart grids, which are systems for managing energy from energy plants through a network and optimizing the demand/supply balance of energy, and energy storage systems. In the field of information communication, information terminals such as smartphones are rapidly penetrating the society, since they easily receive and transmit information. Under these circumstances, energy storage devices, such as capacitors or rechargeable batteries, are required to have high output density, high energy density, and long life to improve the performance of smartphones, hybrid vehicles, electric vehicles, smart grids, and the like, as well as to reduce production cost.
Among the commercialized energy storage devices, lithium-ion rechargeable batteries (commonly called “lithium-ion batteries”), which each use carbon, such as graphite, as an anode and a compound of lithium and transition metals as a cathode, have the highest energy density. However, a lithium-ion rechargeable battery, whose anode is formed of a carbon material, can theoretically intercalate only a number of lithium atoms corresponding to up to ⅙ the number of carbon atoms. For this reason, it is difficult to further increase the capacity, and increasing the capacity requires a new electrode material. Lithium-ion rechargeable batteries are expected to become power supplies for hybrid vehicles or electric vehicles thanks to their high energy density. On the other hand, lithium-ion rechargeable batteries have a problem that when quickly discharged, they cannot discharge a sufficient amount of electricity due to their large internal resistance, that is, their output density is low. For this reason, there is a demand to develop high-output-density, high-energy-density energy storage devices. To satisfy this demand, tin, silicon, and alloys thereof, which can store and release more lithium ions than graphite, are being studied. Tin and silicon can electrochemically store more lithium ions. However, these elements expand their volumes by a factor of as many as about four and repeatedly expand and contract through charges and discharges, resulting in increases in the electrode resistance and reductions in the electrode performance. To prevent reductions in the electrode performance, there have been made various proposals about silicon particles themselves, conductive additives, binders, and current collector substrates.
Non-Patent Literature 1 proposes, as an electrode having long charge-discharge cycle life, a silicon nanotube that contains expansion space and whose outermost surface is coated with silicon oxide. Non-Patent Literature 2 proposes a silicon nanowire which is coated with alumina by atomic layer deposition, as an electrode active material having long cycle life. However, the silicon nanotube needs to be produced through many steps and therefore is not suitable for mass production, nor is the silicon nanowire suitable for mass production. Accordingly, any of the silicon nanotube and silicon nanowire is difficult to provide cheaply. Nor is the coating with alumina suitable for mass production.
To suppress the crack of particles during lithiation (lithium insertion), attempts are being made to produce an anode for rechargeable batteries that includes submicron or less sized silicon particles (commonly called “silicon nanoparticles”), which are easily mass-produced unlike the silicon nanotube and silicon wire. However, even such silicon particles have low electron conductivity and expand during lithiation as well. For this reason, Non-Patent Literature 3 and the like propose forming a composite with graphene to improve electron conductivity and to ensure expansion space. However, a method for producing graphene used to improve conductivity described in Non-Patent Literature 3 is not suitable for mass production. On the other hand, Patent Literature 1 proposes a composite of a battery active material capable of forming an alloy with lithium, such as silicon, and expanded graphite or flake graphite. However, any of expanded graphite and flake graphite has a large particle size and therefore it is difficult to mix and disperse and submicron or less-sized silicon particles uniformly. Non-Patent Literature 4 proposes forming a composite of a graphite nanosheet and silicon particles by immersing expanded graphite in a tetrahydrofuran solution of silicon nanoparticles and polyvinyl chloride and applying ultrasound to the expanded graphite to flake off the expanded graphite. However, a flake-off device using ultrasound is difficult to be scaled up and therefore is not suitable for mass production. For this reason, there is a demand to develop a method for producing an easy-to-mass-produce, cheap conductive additive that is suitable for fine silicon particles.
Patent Literature 2 proposes metal coating for improving the electron conductivity of silicon particles and ceramic coating for suppressing the pulverization of silicon particles caused by expansion during lithiation. Patent Literature 3 proposes means of extending the charge-discharge cycle life by providing silicon particles with a metal oxide coating layer formed of a raw material, such as alkoxide, by sol-gel reaction. Patent Literature 4 proposes silicon nanoparticles coated with a metal oxide coating and dispersed in silicon oxide in order to suppress gas generated by the decomposition of an organic solvent in an electrolyte solution. In any of the proposals, the coating of silicon particles does not reduce the amount of silicon oxide that causes an irreversible lithiation-delithiation capacity during charge-discharge but rather often increases the amount, resulting in a reduction in the initial charge-discharge coulombic efficiency.
To form the anode of a lithium-ion battery using silicon particles as an active material, it is important that the binder be formed of a material capable of enduring the expansion and contraction of the volume during lithium storing/releasing. As the material of the binder, Non-Patent Literature 5 proposes carboxymethylcellulose; Non-Patent Literature 6 proposes sodium alginate; and Non-Patent Literature 7 proposes polyacrylic acid. However, these materials have a problem that any of the polymers thereof does not provide sufficient strength when used in a small amount and reduces the conductivity of the electrode when used in an amount to maintain strength. For this reason, there is a demand to develop a technique that can increase the mechanical strength of the electrode even when a small amount of binder is used.
An electrode in which an electrode layer including silicon particles are formed on a current collector formed of a metal foil expands or contracts due to the storing or releasing of lithium. The current collector receives stress accordingly. Since the electrode layer is not completely uniform, the metal foil as a current collector becomes crinkled ununiformly, thereby flaking off the electrode layer. This increases the electrical resistance of the electrode and reduces the charge-discharge cycle life. For this reason, there is a demand for a current collector or electrode structure that does not become crinkled ununiformly. Non-Patent Literature 8 discloses that stress caused by the lithiation is released by using a flexible current collector formed by evaporating a metal thin-film onto a flexible substrate. However, the metal thin-film disadvantageously cannot cope with the charge or discharge of large current. In order to suppress the distortion or breakage of the anode current collector and thus the deformation of the anode, Patent Literature 5 proposes an electrode including a current collector having thereon multiple protrusions and an alloy-based active material disposed on the protrusions. However, this electrode has a problem that a step of eliminating the distortion of the current collector is required and a problem that the lamination of the active material only on the protrusions increases the number of production steps, as well as the production cost.
To improve the cycle life of an electrode using silicon particles as an active material, Non-Patent Literature 9 proposes an electrolyte solution containing vinylene carbonate as an additive; Non-Patent Literature 10 proposes an electrolyte solution containing fluoroethylene carbonate as an additive. These electrolyte solutions extend the life compared to those not containing these additives. However, the thickness of a solid electrolyte interphase (SEI) layer generated by the decomposition of the electrolyte solution is moderately increased, and the conductivity of the electrode is reduced. That is, the growth of an SEI layer is not sufficiently suppressed. For this reason, there is a demand for an additive which is effective in suppressing the growth of an SEI layer.